The present disclosure relates to pattern formation methods in which patterns are formed by performing exposure twice or more on a single substrate.
In recent years, as the industry continues to progress in miniaturization of semiconductor devices to the utmost limit, it is becoming necessary to form a pattern with a size exceeding a resolution limit of optical lithography. A resolvable minimum pitch is represented by a limiting point in optical theory. Resolution is represented by the Rayleigh equation, i.e., R=k1·λ/NA (where R represents a resolution size, λ represents a light source wavelength, NA represents the numerical aperture, and k1 represents a process factor). For example, the minimum k1 is 0.25 in immersion exposure with water under the conditions where a light source wavelength λ, is 193 nm and the NA is 1.35. Thus, the resolution size R is 35.7 nm, and the limit pitch is 71.4 nm, which is twice as large as the resolution size R. However, a 22 nm generation device requires an interconnect pitch of 64 nm, which is not achieved by conventional exposure techniques. Under such circumstances, a so-called “double patterning” is suggested, in which patterns are formed individually in two exposure steps and then superimposed to double the pattern pitch.
One type of double patterning is a litho-etch-litho-etch (LELE) process in which exposure and etching are repeated twice (see, e.g., M. Maenhoudt et al., Double Patterning Scheme for Sub-0.25 k1 Single Damascene Structures at NA=0.75, λ=193 nm, Proceedings of SPIE, Vol. 5754, pp. 1508-1518 (2005)). A litho-process-litho-etch (LPLE) process is also suggested, in which a manufacturing process is simplified by eliminating an etching step (see, e.g., A. Vanleenhove et al., A Litho-Only Approach to Double Patterning, Proceedings of SPIE, Vol. 6520, pp. 65202F-1-65202F-10 (2007)).